Job creation in the private sector was disappointingly slow in May, with companies adding just 179,000 positions, according to the latest reading from ADP and Moody's Analytics.

Economists in a consensus survey expected ADP's national employment report to show the economy created 215,000 private payrolls in May, down from the prior month's 220,000 figure.

The number could cause economists to ratchet down the projections for Friday's nonfarm payrolls report, which is projected to show an addition of 210,000 positions for the month. Capital Economics said in a note that the ADP report "adds to the downside risk" for its forecast of 230,000 jobs created.

Services accounted for most of the jobs, adding 150,000 to the total, while goods-producing employment increased by 29,000. Even in the services sector, though, there was weakness, with new professional and business positions tumbling to 46,000 from 75,000 in April.

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A flier announcing job openings sits on a table during a career fair at the Southeast Community Facility Commission on May 21, 2014 in San Francisco, Calif.

The total April number was revised lower, from 220,000 to 215,000 positions.

"Taken at face value, I'd say it's disappointing," said Mark Zandi, the chief economist at Moody's, which assembles the data for ADP, told CNBC. "It suggests that monthly job growth is probably still south of 200,00 per month and that's about where we've been for the past three years...But i don't know that I'd take it at face value...My sense is that looking at broader sets of data, underlying job growth is more than 200K."

"While the US economy continues to add jobs at a decent pace, there are still no signs of a new acceleration which at this point of the business cycle remains telling and helps to explain the move in US Treasuries this year compared to the very optimistic economic expectations entering 2014," Peter Boockvar, chief market analyst at The Lindsey Group, said in a note.